Ice detection systems have been used in many different applications. Patent DE 44 37 577 discloses an ice detection device placed under a road for automotive vehicles, to determine whether there is ice on the road. This device includes a transmitter which transmits a first radiation and a second radiation in the direction of a unit incorporated within the road; a receiver which captures the first radiation and the second radiation reflected by a reflective surface; said receiver delivers a first intensity representative of the first captured radiation and a second intensity representative of the second captured radiation; and a computer which determines the presence or absence of ice from the ratio of the first intensity and the second intensity. For example, when this ratio is positive, ice is considered to be on the road; when this ratio is negative, ice is considered not to be on the road.
Detecting ice on aircraft creates different challenges. U.S. Pat. No. 5,500,530 describes a detection device of the same type, but for detecting the presence of ice on an aircraft on the ground. This device detects ice on an aircraft on the ground and not on an aircraft in flight. When an aircraft is in flight, ice accretion does not occur in the same manner on the various parts of the aircraft because of the existence of higher pressure areas and the different speeds and directions of the winds around the aircraft. It is particularly important to detect the first appearance of ice on the wings of an aircraft in flight so that this ice can be removed, because ice accretion can cause the aircraft to stall due to the loss of lift. Such ice detection is easier when the aircraft is on the ground because the appearance of ice occurs more uniformly.
This detector does not detect the apparition of icing conditions anywhere on an aircraft during flight as well as during Taxi, Take off and Landing phases (TTL) at around 0° C. Local variations in pressure coefficients at certain places on the aircraft can cause drops in temperature and allow ice to accumulate on those areas only, without being detected.